Single crystalline materials (both pure and alloyed) may exhibit unique mechanical, electrical, and optical properties. For metallic single crystals, high strength and resistance to creep are demonstrated, which is important for turbine blades. Single crystalline copper and aluminum exhibit better electrical conductivity, which makes possible reduced parasitic heating in electrical power transmission and motors.
In addition to metals, other materials are grown as single crystals for a variety of applications, including radiation detectors, semiconductor electronics, and optical devices. Conventional methods of producing single crystals generally require slow solidification rates and tightly controlled thermal gradients, which lead to energy intensive and costly manufacturing processes. For example, a single finished five-inch semiconductor grade single-crystal silicon wafer can cost thousands of dollars, and a single-crystal turbine blade can cost upwards of $10K. Much of the cost associated with these components is linked to the processing time and energy associated with manufacturing.
Several methods are currently used to produce high purity semiconductor crystals, including float zone melting, the Czochralski process, and the Bridgman and Stockbarger techniques, while other crystal growth methods may be employed for lower purity applications. In addition to crystal growth, purification steps such as zone refining, in which a narrow region of a crystal is melted and moved along the crystal length, are employed to concentrate impurities at one end of the ingot, leaving the bulk purer after each pass. All of these methods require significant time for the growth to take place—several days to weeks—due to the kinetics at the solidification interface.
An area of materials technology that may benefit from further development of single crystal growth processes is magnetocaloric materials. The magnetocaloric effect (MCE) is based on the change in entropy that occurs with magnetic domain alignment as ferromagnetic materials approach their Curie temperature or as paramagnetic materials are magnetized and demagnetized. The MCE is exhibited as a reversible change in temperature of a ferromagnetic material upon the application or removal of a magnetic field. Advanced magnetocaloric alloys based on earth-abundant non-toxic elements are needed to meet the demands of commercial heating and cooling systems. The classic MCE may be greatly enhanced when the Curie temperature is coupled with a structural transition (magnetostructural coupling), and the cooling effect from the structural transformation may be several times that from the magnetic transformation, and are said to exhibit “giant MCE.” Advanced materials are desired that exhibit large magnetostructural coupling effects and large changes in entropy near ambient temperatures at modest fields of 1-2 Tesla. Refrigeration systems based on the magnetocaloric effect have the potential for improvements of 60-100% in performance over conventional gas compression systems.
For a giant MCE, the structural transition is most commonly a martensitic shear transformation with an associated shape change. As a result, there can be severe transformational stresses that need to be accommodated along the habit plain between the parent and shear phases. There is a threshold defined by the mechanical properties of the alloy above which the transformational stresses are accommodated irreversibly (inelastically) and cannot be used in a refrigeration cycle. Texturing of a material can be an effective method of facilitating the accommodation of these stress, thereby lowering the activation energy for the transformation. This may be accomplished by aligning the structural domains along low energy boundaries such that the direction perpendicular to the habit planes is nearly parallel, as in a single crystal or highly textured material. Improved performance of single crystals in magnetic refrigeration applications has been demonstrated (e.g., Kimura, H., Numazawa, T., Sato, M., Ikeya, T., Fukuda, T., and Fujioka, K., “Single crystals of RAlO3 (R: Dy, Ho and Er) for use in magnetic refrigeration between 4.2 and 20 K,” Journal of Materials Science, vol. 32, pp. 5743-5747, 1997). However, powders may be synthesized with a particle size distribution that is less than the characteristic domain size of the MCE material, essentially making single crystal particles. In addition, extensive thermomechanical processing may be employed to produce a high degree of deformation texture. However, both of these methods result in structural alignment that is less perfect than what is found in a single crystal. In short, various industrial applications, ranging from magnetocaloric devices to semiconductor chips, could benefit from the development of an economical process for fabricating single crystal materials.